dt-bindings: net: bluetooth: qualcomm: document VDD_CH1

WCN3990 comes with two chains - CH0 and CH1 - where each takes VDD
regulator.  It seems VDD_CH1 is optional (Linux driver does not care
about it), so document it to fix dtbs_check warnings like:

  sdm850-lenovo-yoga-c630.dtb: bluetooth: 'vddch1-supply' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230617165716.279857-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>